Another way to find your cadence, or at least find a reference cadence, is to pedal at a constant speed (on flat ground) and work out your cadence by working backwards from your speed, wheel circumference and gear combination.

If you have a 28" wheel, let's say the circumference is 2.22 m, or 7.3 ft.
You read your speed to be exactly 35 km/h (22 mph) and your gear is 48-11.
The gear ratio is 4.36:1, so for every pedal revolution, your wheel makes 4.36 revolutions.
35 km/h = 9.7 m/s.
4.36 * 2.22 = 9.7.
So for 1 pedal revolution per second, the speed is 35 km/h.
And 1 rps = 60 rpm!